1. Field of the Invention
The present invention relates to an optical fiber amplifier for amplifying optical signals, specifically, to an optical fiber amplifier which can reduce the amount of active fiber which constructs the optical fiber amplifier, and improve the amplification efficiency as well, by looping a pumping light beam used as an exciting signal in a rare-earth-doped fiber back to the rare-earth-doped fiber through a feedback loop or reflection loop.
2. Discussion of Related Art
Optical communication techniques transmitting information through optical fibers have been developed and are widely being used. The optical communication techniques, which can transmit large amount of information at high speed, are applied for information communications between countries through submarine cables because they do not suffer from signal disturbance or crosstalk due to electromagnetic induction. As multiplex and network techniques for the optical communications have been developed recently, the optical communication techniques gradually enlarge the range of their use to the key communication networks for high-speed broadband multimedia communications including voice and data communications between switches, cable TV or video on demand (VOD).
The optical communication techniques have been improved according to the development of optical signal amplifiers which provide high-speed optical signal transmission and superlong-distance transmission. Recently, there have been actively carried out researches about amplifiers having flat gain wavelength, which is used in wavelength multiplex, and high-gain amplifiers for image distribution techniques.
An early optical signal amplifier converts an optical signal into an electric signal through an avalanche-type photodiode to amplify, and reconverts the amplified electric signal into the optical signal using a laser diode. Present optical signal amplifiers employ rare-earth-doped fibers so that the signal conversion process for optical signal amplification can be omitted. The aforementioned rare-earth-doped fiber is formed in a manner that an active optical fiber is doped with a rare earth ion such as Er, Pr and Nd. When a pumping light beam having a predetermined wavelength is supplied to the rare-earth-doped fiber, stimulated photon having a predetermined wavelength is emitted due to excitation of the rare earth ion, which amplifies the optical signal propagated through a corresponding optical fiber ultimately.
FIG. 1 shows a configuration of a conventional optical fiber amplifier using the rare-earth-doped fiber. Referring to FIG. 1, an optical signal S is coupled to a first optical line 1, and a pumping light beam P is coupled to a second optical line 2, first and second optical lines 1 and 2 being coupled to a multiplexer 3 as its inputs. A third optical line 4 corresponding to the output of multiplexer 3 is connected to a fourth optical line 7 which is the output line, through a rare-earth-doped fiber 5 and isolator 6. In this configuration, optical signal S and pumping light beam P applied through first and second optical lines 1 and 2 respectively are coupled with each other by multiplexer 3 so that they are included together in third optical line 4 corresponding to the output of multiplexer 3.
Optical signal S and pumping light beam P are applied to the rare-earth-doped fiber 5 where pumping light beam P excites rare earth ions doped thereinto, to generate stimulated photon having a predetermined wavelength. This light is introduced into optical signal S and effects optical amplification. Isolator 6 prevents opposite optical signals from being introduced into rare-earth-doped fiber 5, which proceed in a direction opposite to optical signal S and include, for example, pumping light beam from another rare-earth-doped fiber located in the following stage or reflection signal of optical signal S.
The maximum output power of the optical fiber amplifier is determined, depending on the dopant doped into the optical fiber, concentration of the dopant, the length of the doped optical fiber, the wavelength of pumping light, and the output of pumping light. As the optical fiber doped with the rare earth ion is very expensive, it requires to be shortened. However, when the rare-earth-doped fiber becomes shorter, amplification of the optical signal is not sufficiently carried out, and thus an optimum optical signal cannot be obtained.
Furthermore, the pumping light beam as an exciting light in the rare-earth-doped fiber corresponds to a noise signal in terms of the optical signal transmitted through the fiber. Accordingly, to prevent a residual pumping light, which is not consumed but left in the rare-earth-doped fiber, from being transmitted through the optical fiber, the conventional optical fiber amplifier includes a reflection mirror at its output terminal to reflect the pumping light beam. However, the reflection mirror reflects not only the pumping light beam outputted from the rare-earth-doped fiber but also a portion of the optical signal transmitted through the optical fiber. Thus, it may deteriorate the output level of the optical signal.